Optical encoders are used to monitor the motion of, for example, a shaft such as a crank shaft. Optical encoders can monitor the motion of a shaft in terms of position and/or number of revolutions of the shaft. Optical encoders typically use a code wheel attached to the shaft to modulate light as the shaft and the code wheel rotate. In a transmissive code wheel, the light is modulated as it passes through transmissive sections of a track on the code wheel. The transmissive sections are separated by non-transmissive sections. In a reflective code wheel, the light is modulated as it is reflected off of reflective sections of the track on the code wheel. The reflective sections are separated by non-reflective sections. As the light is modulated in response to the rotation of the code wheel, a stream of electrical signals is generated from a photodetector array that receives the modulated light. The electrical signals are used to determine the position and/or number of revolutions of the shaft.
Some conventional code wheels include multiple tracks such as separate position and index tracks. These separate tracks on the code wheel are used to determine the rotational position of the shaft, relative to a fixed rotational position, and to determine the number of revolutions of the shaft. The conventional encoders typically have separate photodiode tracks—one for each of the separate position and index tracks. For example, a conventional encoder with three channel outputs includes an AB photodiode track, an index track, and an index/(“index bar”) track. Alternatively, a conventional encoder with three channel outputs may have an AB track and a combined index-index/track. In any case, separate photodiode tracks are implemented for the AB photodiodes and the index-index/photodiodes.
One disadvantage of using separate position and index tracks is that the size of the code wheel may be larger than it would otherwise be if fewer tracks were implemented. The larger size of the code wheel can affect the type of devices in which the code wheel may be used, or the cost of the code wheel, or both. Additionally, the layout of the separate tracks on the code wheel and photodetectors can limit the resolution of the code wheel. Moreover, the task of precisely aligning the separate tracks with the corresponding photodetector arrays becomes more difficult as the number of tracks increases.